Article In Brief
Aged sedentary mice demonstrated increased hippocampal neurogenesis and hippocampal-dependent learning and memory after receiving plasma from more active aged mice. An elevated liver enzyme in the plasma also seemed to confer a benefit to the sedentary mice.
The long-established neurocognitive benefits of exercise in the elderly can be partly recapitulated in the sedentary by transferring not only serum from the exercisers, but also an elevated liver enzyme found in that serum, according to a paper describing a series of mouse and human studies.
While many previous papers have reported cognitive and other benefits by transferring plasma from young to old animals and humans, the July 10 paper in Science is the first to demonstrate such effects with serum from aged exercisers.
The research team at the University of California, San Francisco (UCSF) administered the plasma from aged mice that exercised to aged sedentary mice. The recipients demonstrated increased hippocampal neurogenesis and hippocampal-dependent learning and memory.
Seeking to find which substance in the serum of exercisers confers the benefits, the researchers screened it, looking for any factors that were elevated in the exercisers. From 12 potential factors, they singled out an enzyme that looked especially promising: glycosylphosphatidylinositol (GPI)–specific phospholipase D1 (GPLD1).
When they administered GPLD1 to aged sedentary mice, it reproduced the neurocognitive benefits seen by giving them serum from the exercisers.
What's more, another experiment described in the paper found that concentrations of GPLD1 in blood were higher in active, healthy older humans compared with their sedentary counterparts.
The results “demonstrate that the beneficial effects of exercise in mitigating brain aging can be conveyed from exercising mice to sedentary mice through plasma transfer,” wrote Victor A. Ansere and Willard M. Freeman, PhD, of the University of Oklahoma Health Sciences Center in commentary accompanying the study in Science.
“The authors also provide compelling evidence that the positive effects of exercise on brain aging are at least partially mediated through hepatic mechanisms and identify a promising target for further study.”
The senior author of the study, Saul Villeda, PhD, assistant professor in UCSF's department of anatomy and the Eli and Edythe Broad Center of Regeneration Medicine and Stem Cell Research, told Neurology Today: “We're just getting started. This was an initial step. Now we're trying to figure out the next steps: How is it changing the brain? Are there additional signals it sends to the brain? We're also interested in whether it works in the context of Alzheimer's disease models.”
The first author of the paper was Alana M. Horowitz, a PhD candidate in Dr. Villeda's lab at UCSF.
Dr. Villeda's group provided a cohort of aged mice continuous access to a running wheel for six weeks, while providing only nesting material to age-matched sedentary controls. Compared with the sedentary mice, those that exercised had greater adult neurogenesis, increased expression of brain-derived neurotrophic factor (BDNF), and improved performance on the radial-arm water maze (RAWM) and contextual fear conditioning paradigms.
When the sedentary aged mice were then administered plasma from aged mice that exercised, they demonstrated improved learning and memory for the platform location. Likewise, during fear conditioning training, old sedentary aged mice receiving plasma from the exercisers demonstrated increased freezing in contextual, but not cued, memory testing.
The group then measured the relative amounts of soluble proteins in the plasma from exercised or sedentary mice, finding 30 factors that increased in the aged mice, 33 that increased in the mature mice, and 12 that increased in both. By analyzing the functions of the 12 factors, they found two involved primarily in metabolic processes, and ultimately settled on the enzyme GPLD1 for further study.
Found abundantly in the liver, GPLD1 cleaves GPI from its attachment to cellular membranes, releasing it into the circulation. Although not previously linked to neurogenesis or cognition, Dr. Villeda's group found it had effects on both.
“In exercised and sedentary aged mice,” the study found, “we observed a significant correlation between increased GPLD1 concentrations in plasma and improved cognitive performance in the RAWM and contextual fear conditioning behavioral tests. Furthermore, we detected an increase in GPLD1 in plasma from active, healthy elderly human individuals relative to their sedentary counterparts. These data identify GPLD1 as an exercise-induced circulating blood factor in aged mice and humans with potential relevance to cognitive function in mice.”
Interestingly, Dr. Villeda's group found that levels of GPLD1 do not decline with age, either in the liver or in the circulation, and that the enzyme does not appear to cross the blood-brain barrier. It did, however, decrease the expression of urokinase-type plasminogen activator receptor (uPAR), which is strongly activated during inflammation, immune responses, injury, or stress. Associated changes were also observed in the coagulation and complement system.
“We saw a dampening of inflammation and increased wound-healing responses,” Dr. Villeda said. But, he added, “By no means are we saying that GPLD1 is the one and only factor that benefits the brain.”
GPLD1 is not the first circulating blood factor shown in previous studies to increase with exercise and to affect the brain. The commentary accompanying the paper noted that exercise increases circulating concentrations of irisin, a myokine that acts directly on the brain.
Another muscle secretory factor, cathepsin B protein, was shown in a 2016 paper in Cell Metabolism to be another mediator of the effects of exercise on cognition.
Ozioma Okonkwo, PhD, associate professor in the University of Wisconsin School of Medicine and Public Health's division of geriatrics and gerontology and the Wisconsin Alzheimer's Disease Research Center, commended the authors for their scientific breakthrough, and for carrying out such a rigorous and impressive array of experiments. However, he noted it would be premature at this time to consider GPLD1 a “deliverable” that could be given to humans in a clinical trial.
Dr. Okonkwo said he expects that GPLD1 will prove to be one of many factors that will eventually lead to a combination treatment for brain aging. He compared it to the experience with BDNF.
“BDNF is one of the most studied and understood drivers of the clinical and cognitive benefits of exercise,” said Dr. Okonkwo. “But we have yet to develop a BDNF drug. That tells you how challenging clinical translation is in this area.”
But, he said, “This is very exciting work; it describes a novel liver-to-brain axis that has not previously been observed.”
Regarding the finding that older humans who exercise have increased levels of GPLD1, Dr. Okonkwo said, “This is a very promising finding. It would have been really reassuring to see not only that their levels are higher, but that they have better clinical outcomes. Hopefully, the authors will look into this important next step in their future studies.”
Dr. Okonkwo was senior author of a paper last year in Neurology involving individuals who carried the APOE4 gene and were in preclinical Alzheimer's disease; those who also carried a variant of the klotho gene, shown to increase their circulating level of the hormone, had less APOE4-related amyloid burden. Klotho has previously been shown to increase after exercise and to improve brain function in aging. “We are trying now to see if we can augment klotho in humans,” he said.
Dena Dubal, MD, PhD, associate professor of neurology who holds the David A. Coulter Endowed Chair in Ageing and Neurodegenerative Disease at UCSF, has been pursuing research into klotho's effects on the brain and discovered it counters brain aging. She holds a patent, with UCSF, on the use of klotho for cognition.
“We would love to know and will be working with Saul [Villeda] on this question; how these drivers of good brain health might be linked to each other,” Dr. Dubal said. “Maybe GPLD1 increases klotho levels. Maybe klotho increases GPLD1. Or is there a common mechanism which all of these factors link to?”
Like GPLD1, she noted, klotho does not cross the blood-brain-barrier despite enhancing the brain in mice. “That's a real conundrum,” Dr. Dubal said. “We are working hard to understand what the messenger is that transfers benefits to the brain.”
David R. Kornack, PhD, associate professor in the department of neuroscience and the Del Monte Institute for Neuroscience at the University of Rochester Medical Center, called the new study “very thoughtful, carefully executed and cautiously interpreted.”
“Like many good studies, however, it raises more questions than it answers,” said Dr. Kornack. “What is it about exercise that causes the liver to release GPLD1 into the bloodstream? It's going to be interesting to look at the type, duration, and intensity of exercise that has the best effect.”
Rudolph Tanzi, PhD, the Joseph P. and Rose F. Kennedy Professor of Neurology at Harvard Medical School and director of the Genetics and Aging Research Unit, was senior author of a paper published in Science in 2018 showing that exercise increased hippocampal neurogenesis in mature mice, improved their cognition, reduced amyloid-beta load, and increased levels of BDNF and other factors. His group also reported that by inducing hippocampal neurogenesis in the mice through genetic and pharmacological means, while simultaneously elevating BDNF levels in the brain, they saw the same benefits of exercise on Alzheimer's disease mice.
Dr. Villeda's group is “injecting plasma to achieve what we achieved,” Dr. Tanzi said. “They're using this GPLD1 pathway to get there. It's very interesting. It's pretty convincing that this is at least one of the factors linking exercise with improved cognition. I don't believe it will be the only one.”
Drs. Villeda, Kornack, and Okonkwo reported no relevant disclosures. Dr. Dubal has consulted for Unity Biotechnology.